Depends on what you have. Look under the active ingredients for Dextromethorphan (HBr). It will usually be per tbsp or 15mL, so divide the amount of Dex (usually either 15 or 30 mg) by the dose amount. Then multiply that by the number of mL in the bottle.

EXAMPLE:
Robitussin Max Strength Cough has 15mg DXM per teaspoon (5mL). So 15/5= 3mg per mL. The bottle is 237mL, so there's 237 * 3 = 711mg of DXM in the whole bottle.

Make sure whatever you're using only has DXM under active ingredients, some of the other drugs in cough syrups can be dangerous (or at least cause massive amounts of vomiting).

I personally recommend checking oneself for OCD at least once every 5 minutes.

Don't worry about the vomiting, that'll only happen at a high dose (and that might have been the shit in the pills I took, it had more than just DXM). And if you do vomit, its not that bad, I was so far out of my senses that vomiting wasn't unpleasant at all, it just relieved my terrible stomachache. Oh and the trip is crazy shit, you won't know it til you try it.

I personally recommend checking oneself for OCD at least once every 5 minutes.

Just get yourself a bottle of cough syrup that only has DXM under active ingredients and you should be good to go. If you don't like the idea of cough syrup, you can get Robitussin Gelcaps, 15mg of DXM each, no extra chemicals. They come 20 to a bottle so just down a whole bottle. It'll work.

I personally recommend checking oneself for OCD at least once every 5 minutes.

I've only had 2 dxm trips so I don't know everything lol, but I like the sucrets extraction method.

Crush the sucrets, dump them into a big bowl of warm water and let them dissolve into the bottom. You get this dxm powder that sinks to the bottom and once it's all settled just careflly pour out as much water as possible. Mix the remaining ingredient into a drink (actually doesn't taste that bad) and drink slowly, too fast can make you a little nauseous.

Sucrets are cheaper than robogels and taste much better then syrup so its kind of a win-win situation.